1. Field of the Invention
The present invention relates to a printer provided with a curl straightening device capable of straightening a curled recorded sheet.
2. Description of the Related Art
A conventional printer will be described, taking as an example the thermal transfer printer proposed by the applicant in Japanese Unexamined Patent Publication No. 8-174947.
The conventional thermal transfer printer is described with reference to FIGS. 5 and 6. FIG. 5 is a sectional view of the main portion of the conventional printer, while FIG. 6 is a schematic view illustrating the operation of a curl straightening device of the conventional printer. A print platen 20, having a print surface 20a, is disposed in the body (not shown) of a thermal transfer printer S of FIG. 5. A thermal head 21, which can freely come into contact with and separate from the print platen 20, is disposed upwardly of the print platen 20 so as to oppose it. An ink ribbon (not shown) and a sheet 2 are supplied between the print platen 20 and the thermal head 21, with the sheet 2 being transported in the direction of arrow A.
The thermal head 21 is disposed at a carriage 22 that can freely reciprocate along the print platen 20 by means of a drive source (now shown). The carriage 22 carries a ribbon cassette provided with an ink ribbon (not shown).
A sheet feed mechanism, including a sheet feed roller 23 and an auxiliary roller 24 rotatably in contact with the upper outer peripheral surface of the sheet feed roller 23, is disposed at the upstream side in the sheet 2 feeding direction, that is at the right side of the print platen 20 of FIG. 5. The sheet 2 is nipped between the sheet feed roller 23 and the auxiliary roller 24, which rotate to transport the sheet 2 in the direction of arrow A, that is along the print surface 20a of the print platen 20. At the downstream side or left side of the print platen 20 of FIG. 5, recorded sheet 2 is nipped between a sheet discharge guide 25 and a sheet discharge roller 26 (which make up a sheet discharge mechanism). Then, it is discharged to a downstream side curl straightening device 27 as a result of the rotation of the sheet discharge roller 26.
In the curl straightening device 27, a curl straightening head 28, having a base 28a as shown in FIG. 6, has a guide roller 28b below the base 28a and a roller 28c at the upper end of the base 28a, so that the head 28, supported by a guide shaft 29, can freely reciprocate along the guide shaft 29. Supporting frames 30 are integrally formed at both ends of the guide shaft 29 in order to support the guide shaft 29, with a cam surface 30a being formed at the front side of the supporting frames in the longitudinal direction thereof.
A wide-width protrusion 30b is formed at the center portion of the cam surface 30a in the longitudinal direction thereof. Recesses 30c, being lower in height than the protrusion 30b, are formed on the left and right ends of the protrusion 30b. Inclined surfaces 30d are formed continuously between their respective recesses 30c and the protrusion 30b so as to connect two different levels.
When the sheet 2, which has been subjected to recording, is waiting to be supplied to the curl straightening device 27, the curl straightening head 28 is either at the left end side or the right end side of the guide shaft 29, and the guide roller 28b is in contact with either one of the recesses 30c in the cam surface 30a. The curl straightening head 28 rotates in the direction of arrow Y of FIG. 5, with the guide shaft 29 as rotational center, and the roller 28c separates from the lower surface of the supporting platen 31, forming a gap between the roller 28c and the supporting platen 31.
When the sheet curl straightening device 27 is in the aforementioned waiting state, the sheet 2, which is curled as a result of printing using a thermal transfer printer or the like, is set in the gap between the supporting platen 31 and the curl straightening head 28, as a result of being transported from the sheet feed mechanism, such that the print surface opposes the supporting platen 31.
A motor 32, being a drive source for the curl straightening head 28 and being different from the drive source (not shown) for the carriage 22, is mounted to the lower surface of the left end of supporting frame 30. Driving the motor 32 causes a pulley 33 to rotate, and a rope 34 wound upon the pulley 33 to move. The tension in the rope 34 causes the curl straightening head, mounted to the rope 34, to start moving, for example, from the left end side to the right end side.
Movement of the curl straightening head 28 causes the guide roller 28b to move, for example, up the left inclined surface 30d from the left recess 30c to the protrusion 30b of the cam surface 30a. This causes an end of the curl straightening head 28 to rotate in the direction of arrow X with the guide shaft 29 as rotational center, whereby the roller 28c is press-contacted with a predetermined press-contact force against the sheet 2 set at the lower surface of the supporting platen 31.
Then, the roller 28c of the curl straightening head 28 moves in the direction of arrow B as it strokes the back side, or the side opposite the print surface 2a, of the sheet 2 to eliminate the curl of the sheet 2.
When the curl straightening head 28 reaches the right end of the guide shaft 29, the guide roller 28b of the curl straightening head 28 rolls down along the right inclined surface 30b of the cam surface 30a and arrives at the right recess 30c. When the guide roller 28b is at the recess 30c, the curl straightening head 28 rotates in the direction of arrow Y with the guide shaft 29 as rotational center, whereby the roller 28c separates from the supporting platen 31, and thus set in a waiting state.
When a predetermined amount of the sheet 2, curled after printing, is transported in the direction of arrow A to the curl straightening device 27 in the aforementioned waiting state, the curl straightening head 28 moves to the left from the right end along the guide shaft 29, causing the roller 28c to press-contact the supporting platen 31 again and move towards the left while stroking the sheet 2 supported by the supporting platen 31. The above-described operations are repeated in order to straighten the curled sheet 2 by stroking the back side thereof by the roller 28c.
Such a conventional printer S is very costly, since separate drive sources must be provided for the moving the curl straightening head 28 and the carriage 22. In other words, the conventional printer S requires a motor 32 as a drive source for reciprocating the curl straightening head 28 of the curl straightening device 27 and another drive source (not shown) for moving the carriage 22 having mounted thereto the thermal head 21 serving as a recording head for the printer S.
In addition, the sheet feed mechanism including the sheet feed roller 23 or the like, the carriage 22, and the sheet discharge mechanism including the sheet discharge roller or the like, and the curl straightening device 27 are disposed successively from the upstream side to the downstream side in the sheet feeding direction. Therefore, when an attempt is made to discharge the sheet S out the printer S through the sheet discharge guide 25 and the sheet discharge roller 26 of the sheet discharge mechanism, the downstream curl straightening device 27 blocks the discharge of the sheet, preventing reliable sheet discharge.
When the sheet discharge mechanism does not successfully discharge the sheet 2 out the printer S, the sheet 2 must be taken out from the curl straightening device 27 by pushing the trailing end of the sheet 2 with some kind of member or by pulling the front end of the sheet 2 discharged from the curl straightening device 27, thus making it troublesome to handle the sheet 2.
The curl straightening head 28 rotates in the direction of arrow X or arrow Y of FIG. 5 as a result of the movement of the lower guide roller 28b from either one of the recesses 30c to the protrusion 30b, or from the protrusion 30b to either one of the recesses 30c. Thus, variations in parallelism between the surface of the supporting platen 31 and the roller 28c occur as a result of, for example, accumulation of component part errors, such as those of the base 28a of the curl straightening head 28, the guide roller 28b, or the roller 28c. This prevents the roller 28c from rolling parallel to the supporting platen 31 while stroking the sheet 2, so that the curled sheet 2 cannot be properly straightened.